1. Field of the Invention
The present invention relates generally to a magnetic encoder and, more particularly, to technology for compensating output signals of a magnetic encoder.
2. Description of the Related Art
In various industrial fields, sensors for detecting the rotational angular velocity and location of a rotating object are required. For this purpose, an optical encoder and a magnetic encoder are chiefly used.
An optical encoder has slots rotating in synchronization with a rotating object and passing between an optical source such as laser and an optical receiver, and can generate output signals for detecting the rotational angular velocity and rotational position of an object. The output signal is a pulse train, thus rotational angular velocity and rotational position can be relatively accurately detected by counting pulses and using the number of pulses per hour, and relatively high rotating speeds can be detected. However, the optical encoder is vulnerable to a change in temperature and dust, cannot be easily fabricated in a small size, and is expensive.
In contrast, a magnetic encoder can generate an output signal for detecting the rotational angular velocity and rotational position of a rotating object using induced voltage that is induced by a piece of magnetic material rotating in synchronization with a rotating object. Accordingly, the magnetic encoder, having a relatively simple configuration, is less sensitive to varying temperature and dust occurring in operating environment, and can be fabricated in a small size.
An output signal from the magnetic encoder are composed of a pair of sinusoidal waves: a sine wave and a cosine wave, which are based upon the induced voltage of a rotating magnetic field. A rotational position and a rotational speed can be detected based upon the phases of the two waveforms. However, although the magnetic encoder is inexpensive, two output signals of the magnetic encoder have low resolution because they are vulnerable to noise, phase drift, DC offset, amplitude variation, and waveform distortion.
Recently, there have been several attempts to achieve level of precision of the optical encoders, from the magnetic encoders, having low cost to make and decent mechanical performance, by compensating output signals of the much less expensive magnetic encoders and converting the compensated output signals into a pulse train, as from the optical encoders.
For example, a method using a Kalman filter widely used to eliminate noise from sampling data in a high noise environment, or a method using an observer has been proposed, but a problem arises in that phase drift is unsolvable or an excessive time is required to settle.